Scholarly article on topic 'A Novel Process Chain for the Production of Spherical SLS Polymer Powders with Good Flowability'

A Novel Process Chain for the Production of Spherical SLS Polymer Powders with Good Flowability Academic research paper on "Materials engineering"

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{polymers / "wet grinding" / "downer reactor" / "dry particle coating" / "selective laser sintering"}

Abstract of research paper on Materials engineering, author of scientific article — Jochen Schmidt, Marius Sachs, Christina Blümel, Bettina Winzer, Franziska Toni, et al.

Abstract Rapid prototyping has been applied so far in the production of special parts at low piece numbers. Currently, rapid prototyping gradually is transferred to additive manufacturing opening new applications. Especially selective laser sintering (SLS) applying polymer powder systems is promising. In the case of SLS basically only polyamide is available as an optimized powder material showing satisfying behavior during processing. Other types of polymer powders produced by cryogenic grinding show poor powder flowability as well as an unfavorable particle habitus resulting in poor device quality. In fact, it is challenging to produce laser sintering powders with small particle size, good flowability and processability. Within this account we present a novel process route for the production of spherical polymer micron-sized particles of good flowability. The feasibility of the process chain is demonstrated for polystyrene (PS) and poly butylene terephthalate (PBT) and the increase of powder flowability after the consecutive process steps has been monitored using a tensile strength tester. The influence of particle habitus and surface functionalization on powder flowability and its properties is discussed. In a first step polymer microparticles are produced by a wet grinding method at reduced temperatures. By this approach the mean particle size and the particle size distribution can be tuned between a few microns and several 10 microns and adapted to specific needs. The dependencies of mean product particle size, particle size distribution and grinding kinetics on stressing conditions, system composition and especially process temperature (increase of the brittleness of the polymer vs. increase of dampening of grinding bead motion with decreasing temperature) will be extensively discussed for the polymers PS, PBT, poly oxo methylene (POM) and Poly ether ether ketone (PEEK). The comminution products obtained typically consist of microparticles of irregular shape and poor powder flowability, i.e. these intermediate products are cohesive and thus will show poor SLS processability. An improvement of flowability of the ground polymer particles is achieved in a second step by changing their shape. The irregular particles are rounded using a heated downer reactor. It will be demonstrated that the ‘degree’ of rounding can be controlled by changing the temperature profile or the residence time. To further improve the flowability of the cohesive spherical polymer microparticles nanoparticles are adhered to the microparticles’ surface in a third step. The improvement of powder flowability after the consecutive process steps is remarkable: rounded and dry-coated PS powders exhibit a strongly reduced tensile strength (by a factor 5) in comparison to the tensile strength of the edged PS comminution product. The improved flowability and packing behavior of the polymer powders open new options in SLS processing including the usage of much smaller polymer beads.

Academic research paper on topic "A Novel Process Chain for the Production of Spherical SLS Polymer Powders with Good Flowability"

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Procedía Engineering 102 (2015) 550 - 556

Procedía Engineering

www.elsevier.com/locate/procedia

The 7th World Congress on Particle Technology (WCPT7)

A novel process chain for the production of spherical SLS polymer

powders with good flowability

Jochen Schmidta*, Marius Sachsa, Christina Blümela, Bettina Winzera, Franziska Tonia,

Karl-Ernst Wirtha and Wolfgang Peukerta

aInstitute of Particle Technology (LFG), Cauerstr. 4, D-91058 Erlangen, Germany

Abstract

Rapid prototyping has been applied so far in the production of special parts at low piece numbers. Currently, rapid prototyping gradually is transferred to additive manufacturing opening new applications. Especially selective laser sintering (SLS) applying polymer powder systems is promising. In the case of SLS basically only polyamide is available as an optimized powder material showing satisfying behavior during processing. Other types of polymer powders produced by cryogenic grinding show poor powder flowability as well as an unfavorable particle habitus resulting in poor device quality. In fact, it is challenging to produce laser sintering powders with small particle size, good flowability and processability. Within this account we present a novel process route for the production of spherical polymer micron-sized particles of good flowability. The feasibility of the process chain is demonstrated for polystyrene (PS) and poly butylene terephthalate (PBT) and the increase of powder flowability after the consecutive process steps has been monitored using a tensile strength tester. The influence of particle habitus and surface functionalization on powder flowability and its properties is discussed. In a first step polymer microparticles are produced by a wet grinding method at reduced temperatures. By this approach the mean particle size and the particle size distribution can be tuned between a few microns and several 10 microns and adapted to specific needs. The dependencies of mean product particle size, particle size distribution and grinding kinetics on stressing conditions, system composition and especially process temperature (increase of the brittleness of the polymer vs. increase of dampening of grinding bead motion with decreasing temperature) will be extensively discussed for the polymers PS, PBT, poly oxo methylene (POM) and Poly ether ether ketone (PEEK). The comminution products obtained typically consist of microparticles of irregular shape and poor powder flowability, i.e. these intermediate products are cohesive and thus will show poor SLS processability. An improvement of flowability of the ground polymer particles is achieved in a second step by changing their shape. The irregular particles are rounded using a heated downer reactor. It will be demonstrated that the 'degree' of rounding can be controlled by changing the temperature profile o r the

* Corresponding author. Tel.: +49-9131-8529408; fax: +49-9131-8529402. E-mail address: jochen.schmidt@fau.de

1877-7058 © 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.Org/licenses/by-nc-nd/4.0/).

Selection and peer-review under responsibility of Chinese Society of Particuology, Institute of Process Engineering, Chinese Academy of Sciences (CAS) doi:10.1016/j.proeng.2015.01.123

residence time. To further improve the flowability of the cohesive spherical polymer microparticles nanoparticles are adhered to the microparticles' surface in a third step. The improvement of powder flowability after the consecutive process steps is remarkable: rounded and dry-coated PS powders exhibit a strongly reduced tensile strength (by a factor 5) in comparison to the tensile strength of the edged PS comminution product. The improved flowability and packing behavior of the polymer powders open new options in SLS processing including the usage of much smaller polymer beads.

©2015TheAuthors.PublishedbyElsevierLtd.Thisis an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Selection and peer-review under responsibility of Chinese Society of Particuology, Institute of Process Engineering, Chinese Academy of Sciences (CAS)

polymers; wet grinding; downer reactor; dry particle coating; selective laser sintering

1. Introduction

Currently rapid prototyping gradually is transferred to additive manufacturing whereas few years ago it only has been applied in the production of special parts at low piece numbers. New applications appear at the horizon, however, there are still several restrictions that need to be resolved. Besides limitations that are due to features of the selective laser sintering (SLS) process also the limited choice of commercially available polymer powder materials is unfavorable. Today basically only polyamide (PA)-based laser sintering powders are applied which make up 95% of the market [1]. These PA powders exhibit good processability and are e.g. optimized with respect to their flowability and particle size distribution. Micron-sized polymer powders produced by cryogenic grinding have been evaluated [2]. Only parts of poor quality have been obtained which can be ascribed to poor flowability, low bulk density and adverse habitus of the grinding products.

In this paper a process chain for the production of micron-sized spherical polymer powders which is suitable for a variety of polymer materials like polystyrene (PS), poly ether ether ketone (PEEK), poly oxy methylene (POM) and poly butylene terephthalate (PBT) is discussed [3].

The first step is wet grinding of polymers at reduced temperatures [4]. Mean product particle sizes of some microns can be obtained. It will be demonstrated that this alternative approach is feasible. So far, for the comminution of polymers typically is accomplished by applying rotor impact mills, cutting mills or jet mills [4-7]. High impact velocities and low process temperatures are favorable, as the viscous dissipation is strongly reduced at these load conditions. Due to plastic or viscoelastic material behavior of polymers grinding is known to be costly and energy-intensive, if product particle sizes in the micron size range are desirable [5-11]. From our point of view the wet grinding method is advantageous due to the high stress numbers that can be realized in stirred media mills [12]. The dependencies of product characteristics on process parameters in wet grinding will be discussed. Optimum process conditions with respect to temperature and solvent viscosity exist: with decreasing process temperature the brittleness of the polymer increases which improves grinding efficiency. However, the solvent viscosity increases with decreasing process temperature, too, which causes a dampening of the grinding bead motion. In consequence the grinding efficiency decreases.

The second process step is rounding of the comminution product in a heated downer reactor. Spherical polymer particles showing better flowability as compared to the grinding product are obtained at appropriate operation conditions. The flowability of the rounded product particles is remarkably increased by a dry particle coating process [13, 14]. The increase of flowability has been characterized by tensile strength measurements of the powders [15].

2. Materials and Methods

Grinding experiments have been performed using a stirred media batch mill PE075 (Netzsch). Zirconia grinding beads (Tosoh) sized dGM = 2.0 mm (pGM = 6,050 kg/m3) have been applied. Stress intensity SE (see equation (1)) can be altered by changing the stirrer tip speed vtip between 3.6 m/s and 7.1 m/s and by changing grinding media size dGM. Stress intensity SE is proportional to the kinetic energy of a grinding bead [12].

SE ^ 4M • vtip -PGM (i)

As feed materials PS (Carl Roth, x50,3 = 3 6 3 ^m), PEEK (Vicote 705 (Victrex), x50,3 = 21.5 ^m), POM (Hostaform C9021 (Ticona), x50,3= 50 ^m) and PBT (Ultradur B4520 HS (BASF), x50,3 = 61.3 ^m) have been used at mass concentrations of 6.5 to 18.5 wt.-%. The aforementioned polymer powders have been obtained by pre-comminution of N2 (l) cooled commercially available granules of 3 to 4 mm size using a rotor mill (Pulverisette14, Fritsch). Approximately 1,500 g grinding beads have been introduced into the grinding chamber (of about. 650 mL total volume) for each experiment. The wet grinding process has been studied for ethanol (95%, VWR) and n-hexane (> 99%, Carl Roth) for PS, POM and PEEK. Mass specific comminution energy was determined for wet grinding of PBT in ethanol, butanol and n-octane (>= 95%, VWR). In these experiments instead of the aforementioned stirred media mill PE75 a self-made stirred media mill was used that was equipped with a torque gauge to allow for determination of comminution energy consumed. Process temperature has been varied between -80°C and 30°C using the thermostat device unistat905w (Huber).

For dry coating fumed silica R1 (Evonik) with a primary particle size of 7 nm has been applied at 1 wt-% using a tumbling mixer (T2F, Bachofen).

Particle size distributions have been determined by laser diffraction particle sizing using a Mastersizer 2000 / Hydro 2000S (Malvern).

Flowability measurements were conducted using a Zimmermann tensile strength tester (for details see [15]). 3. Results

3.1. Wet grinding

The influence of process temperature on the wet grinding behavior of PS in ethanol at constant stressing conditions is outlined in Figure 1. The influence of temperature-dependent material parameters such as solvent viscosity on the stress intensity (see equation (1)) has been considered (for details see [4, 16]).

■ 1

▲ A 80°C, SE = 2.47E-03 J

• ■ -40°C, SE = 2.56E-03 J

\ f A • Y V.,= 0°C, SE = 20°C, SE = 7.1 m/s, dGM 2.56E-03 J 2.60E-03 J = 2.0 mm

• ■

< 4 ▼

• ▼

............■ T

• ¥

................ g • i * 4 A

I ■

10 100 1000 process time / minutes

Fig. 1. Influence of process temperature on grinding kinetics (x50,3 in dependence on process time) of PS at similar stressing conditions.

Obviously the decreasing process temperature from 20 °C to -40°C increases grinding kinetics, however, upon further decrease of temperature (to -80°C) the kinetics become worse. The increase of comminution kinetics with temperature decrease is due to an increase in brittleness of the polymer, i.e. grinding efficiency increases (c.f. mean

product particle of about 10 microns after processing for about 100 minutes at -40°C as compared to about 240 minutes at 20°C) . Remarkably, product particle sizes x50,3 as small as approximately 2 ^m are achievable by the methods proposed. This particle size is in the order of magnitude of the plastic zone according to Dugdale which can be considered as the grinding limit of the material under consideration (for details see [4]). Upon further reduction of the process temperature the positive effect of increasing brittleness is counterbalanced: grinding becomes less efficient due to dampening of the grinding bead motion [16] (c.f. viscosity of ethanol: 4.7 mPas (-40°C) vs. 18.5 mPas (-80°C)). Thus, one can conclude that an optimum with respect to grinding kinetics exists which will be determined by the T-dependent mechanical properties of the polymer under consideration as well as the T-dependent solvent viscosity. Similar dependencies are observed for PEEK and POM.

Figures 2 and 3 exemplify the effect of solvent viscosity and process temperature for grinding of PEEK and POM, respectively, at similar stressing conditions. Ethanol and hexane have been used as solvents. Again increased grinding kinetics are observed in the low-viscous hexane system. Also the grinding bead dampening phenomenon which decreases grinding kinetics becomes obvious in the ethanol systems at -80°C. for PEEK and POM limiting particle sizes of about 4 ^m and 5 ^m are observed independent of solvent choice. Remarkably, the absolute values of the experimentally observed grinding limit for the aforementioned polymers is only about 1/10 of the value estimated by the extension of the plastic zone. The grinding limit of polymers obtained by the wet grinding method are well below typical product particle sizes obtained e.g. by impact mills. The underlying mechanisms are currently under investigation.

The influence of stress intensity [4] on grinding kinetics has been reported previously. In short, as anticipated higher stress intensities lead to an increase in grinding kinetics.

28 E 24

® <c N 16

"3> a>

o 12 t

100 1000 process time / minutes

c 1 H n= 3.5 ■ n= 3.5 mPas (ethanol). mPas (ethanol). mPas (ethanol). mPas (hexane). -30°C -30°C -80 °C -80 SE = 2.56 SE = 2.56 E-03 J E-03 J E-03 J E-03 J

» 11 = 18.5 n= 1.3 SE = 2.46 SE = 2.53

u • □ ■ □

▼ • ▼ • ■ □ a

▼ -8

▼ C b □

▼ » »» ■ a 1 ■

Fig. 2. Influence of process temperature and solvent on grinding kinetics (x50,3 in dependence on process time) of PEEK at similar stressing

conditions.

▼ ■ - 30°C (ethanol), SE 2 56 E-03 J

Y • -80°C (ethanol), SE = 2.47 E-03 J ▼ -80°C (hexane), SE = 2.53 E-03 J v. = 7.1 m/s, d„„ = 2.0 mm bp GM

• 1 • I ■ • • • ••

■ ▼ ■

■ ■ •

▼ # ■

▼ ■ ■ ■ m -

100 1000

process time / minutes

Fig. 3. Influence of process temperature and solvent on grinding kinetics (x503 in dependence on process time) of POM at similar stressing

conditions.

Using low-viscous solvents is also advantageous with respect to comminution energy needed to attain a desired product particle size. Figure 4 summarizes the mean particle sizes x50 3 obtained for grinding PBT in the solvents butanol, ethanol and n-octane and the corresponding consumed mass-specific comminution energies. In all experiments reported in Figure 4 stress intensities SE of about 2.6 mJ and a feed mass concentration of about 18 wt-% were applied. The process temperature was 15 to 20 °C 1000

s „„

'€ га

Fig. 4. Influence of solvent on mass specific comminution energy and corresponding product particle sizes x50,3 for wet grinding of PBT ( SE =

2.6 mJ, 15 ... 20 °C).

The effect of solvent is tremendous. Product particle sizes differing for more than an order of magnitude are observed for same mass specific energies. Whereas in the butanol-based system there is only little progress in size

octane ethano

Ж butanol

* ж *>

• о О о CP с >Q

• • г*

100 1000 10000 mass specific comminution energy / kJ/kg

reduction, grinding in n-octane is very efficient. Thus, appropriate choice of solvent allows for large savings of energy. In terms of mass specific comminution energies the wet grinding process is comparable to the energies needed for size reduction in dry grinding of polymers [1].

3.2. Rounding of polymers in a downer reactor

The second step of the process chain consists of a rounding process that is described in detail elsewhere [3]. It is performed in a downer reactor equipped with a three stage heating system. The dried comminution product particles obtained by the wet grinding process described in the previous section particles were dispersed using a powder disperser with brush. The aerosol was fed into the downer reactor as primary gas flow via an inner tube. A secondary gas flow (sheath gas) is applied to guards the aerosol in the center of the downer reactor cross section. Nitrogen is used as carrier and sheath gas. Appropriate temperature setting of the heating system allows for melting and rounding and subsequent solidification of the polymer particles. It has been proven that the particle size distributions Q3 of the feed and the rounded product do not differ significantly, i.e. coalescence of the molten particles is minimized by the setup. Figure 5 outlines the effect of residence time on the degree of rounding for PS particles of x50 3 of about 10 ^m. The temperature setting of the three stage heating system was 200 °C / 150 °C / 100 °C. A solid concentration (vol./vol.) of 2*10-8 was applied. Residence time was altered by changing primary and secondary gas flow. From the SEM pictures taken for samples it can be concluded that under the aforementioned conditions a residence time of 2.7 s is sufficient to obtain spherical particles. For smaller residence times melting and coalescence are not accomplished. The residence time necessary for (complete) rounding of the irregular comminution products

Fig. 5. Influence of residence time on rounding of PS particles. (a) left: 0.6 s residence time; (b) right: 2.7 s residence time.

3.3. Dry coating

Remarkable improvement of flowability in terms of reduction of tensile strength can be achieved by dry coating applying fumed silica R1 at 1 wt-% (see Table 1). Particles obtained by wet grinding show a tensile strength of approximately 25 Pa which corresponds to poor flowability. The rounded particles are less cohesive. The tensile strength of the powders obtained after dry coating decreases remarkable. Excellent flowability is observed for the rounded and dry coated PS particles. A tensile strength of about 4 Pa is observed which is superior as compared to the dry coated irregular comminution product. It is obvious that besides roughness also the particle shape has a significant influence on powder flowability.

Table 1. Change of tensile strength of powders along the process chain.

Ground PS Rounded PS Ground PS Rounded PS

+ 1 wt-% R1 + 1 wt-% R1

Tensile strength / Pa 25 12 10 4

4. Conclusions

A novel process chain for the production of polymer microparticles with good flowability has been established [3]. The process steps wet grinding, rounding and dry coating are scalable, i. e. the process chain can be transferred to the plant scale. It has been demonstrated that proper choice of solvent and process temperature is crucial with respect to advantageous grinding kinetics and energy saving. A remarkable improvement of powder flowability can be realized by rounding and dry coating. Polymer powders of improved flowability are e feasible in SLS processing.

Acknowledgements

This study has been supported by Deutsche Forschungsgemeinschaft (DFG) within the framework of the collaborative research center CRC 814 "Additive Manufacturing" (projects A1, A2, A3). Financial support is gratefully acknowledged.

References

[1] R. D. Goodridge, C. J. Tuck, R. J. M. Hague, Progress in Materials Science 57 229-267 (2012).

[2] D. Rietzel, F. Kühnlein, D. Drummer, RTejournal - Forum für Rapid Technologie 6 (2011) (urn:nbn:de:0009-2-31138).

[3] J. Schmidt, M. Sachs, C. Blümel, B. Winzer, F. Toni, K.-E. Wirth and W. Peukert, A novel process route for the production of spherical SLS polymer powders with small size and good flowability. Powder Technol. (submitted) (2014).

[4] J. Schmidt, M. Plata, S. Tröger, W. Peukert, Powder Technol. 228 84-90 (2012).

[5] M. Wilczek, J. Bertling, D. Hintemann, Optimised technologies for cryogenic grinding. Int. J. Miner. Process., 74S, 425-434 (2004).

[6] K. Schönert, Das Zerkleinern von Polymeren. DECHEMA-Monographien, 79,. 67-88 (1976).

[7] W. Peukert, L. Vogel, Comminution of polymers - An example of product engineering. Chem. Eng. Techn. 24, 945-950 (2001).

[8] M. Juhnke, R. Weichert, Zerkleinerung weicher Materialien ohne Verunreinigung der Produkte durch die Mahlkörper. Chem.-Ing.-Tech., 77, 90-94 (2005).

[9] A. Weber, U. Teipel, H. Nirschl, Comparison of comminution by impact of particle collectives and other grinding processes. Chem. Eng. Technol., 29, 642-648 (2006).

[10] M. Liang, C. Lu, Y. Huang, C. Zhang, Morphological and structural development of poly(etherether ketone) during mechanical pulverization. J. of Appl. Polymer Sci., 106, 3895-3902 (2007).

[11] S. Molina-Boisseau, N. Le Bolay, Fine grinding of polymers in a vibrated bead mill. Powder Technol., 105, 321-327 (1999).

[12] A. Kwade, Determination of the most important grinding mechanisms in stirred media mills by calculating stress intensity and stress number. Powder Technol., 105, 382-388 (1999).

[13] H. Rumpf, Die Wissenschaft des Agglomerierens. Chem. Ing. Tech. 46 1-11 (1974).

[14] M. Linsenbühler, K.-E. Wirth, An innovative dry powder coating process in non-polar liquids producing tailor-made micro-particles. Powder Technol. 158 3-20 (2005).

[15] A. Schweiger, I. Zimmermann, A new approach for the measurement of the tensile strength of powders Powder Technol. 101 7-15 (1999).

[16] C. Knieke, C. Steinborn, S. Romeis, W. Peukert, S. Breitung-Faes, A. Kwade, Nanoparticle production with stirred-media mills: opportunities and limits. Chem. Eng. Technol. 33, 1401-1411 (2010).

[17] M. Sachs, J. Schmidt, F. Toni, C. Blümel, B. Winzer, W. Peukert, K.-E. Wirth, Rounding of irregular polymer particles in a downer reactor. The 7th World Congress on Particle Technology (WCPT7), Proceedings (submitted).